This invention pertains to efficient reflective coupling of collimated light into the input end of a light guide.
FIG. 1 schematically depicts the way in which light is conventionally emitted into the input end of a light guide of the type disclosed in U.S. Pat. Nos. 4,615,579; 4,750,798; 4,787,708; 4,834,495; or, 4,850,665. An elongate (i.e. non-point) light source such as an arc-type metal halide light bulb 10 mounted in a socket 12 is slidably inserted through an aperture 14 in a paraboloidal collimator 16. The circumferential rim at the wide end of collimator 16 is flanged as shown at 18. A mating circumferential flange 20 is provided around the input end of hollow prism light guide 22. Flanges 18, 20 are connected together to mechanically couple collimator 16 to the input end of light guide 22. When light bulb 10 is energized, light rays emitted by the bulb""s arc portion 24 are reflected and collimated by collimator 16 toward the input end of light guide 22 which confines the rays and distributes them uniformly along the light guide.
Prism light guides work best when the input light is collimated within a half angle of about 30xc2x0. This requirement limits the range of suitable light sources, because the efficiency with which light can be emitted into the input end of the light guide decreases rapidly as the size of the light bulb increases. Arc-type metal halide light bulbs of the type shown in FIG. 1 are reasonably practical light sources for illuminating light guides, due to their high efficiency, compact size and reasonably long lamp life.
The maximum efficiency with which prior art paraboloidal collimator 16 can couple collimated light into light guide 22 depends on the ratio of the light guide""s diameter xe2x80x9cDxe2x80x9d, to the length xe2x80x9cLxe2x80x9d of the light bulb""s arc. For a typical prior art D:L ratio of about 6:1, reflector efficiencies of about 70% can be achieved. Thus, about 30% of the light emitted by light bulb 10 is typically lost, in the sense that it does not enter the input end of light guide 22. It is desirable to reduce the ratio D:L, since this would enable the use of larger, higher output, more efficient metal halide arc lamps. But, even modest reductions in the ratio D:L substantially reduce the efficiency with which light emitted by the light bulb can be collimated and coupled into the input end of the light guide. For example, a D:L ratio of about 4:1 typically yields a light guide input coupling efficiency of only about 50%, meaning that about 50% of the light emitted by the light bulb is lost, in the sense that it does not enter the input end of the light guide as collimated light.
This invention overcomes the coupling efficiency problems associated with conventional light guide systems. For example, the invention permits efficient collimation and coupling of light emitted by a one kW metal halide light bulb having a 12,000 hour rated life into a 25 cm diameter light guide. Such systems are advantageous in general lighting situations in which high efficiency linear lighting is required and in which maintenance of multiple point source or fluorescent fixtures is problematic.
The invention provides a reflector for reflecting light emitted by an elongate light source into the input end of a light guide having a light guide diameter xe2x80x9cDxe2x80x9d. All embodiments of the invention incorporate a collimating reflector having a narrow apertured end through which the light source is insertably positionable; and, a wide apertured end having a diameter exceeding xe2x80x9cDxe2x80x9d, through which light is emitted into the light guide. All embodiments of the invention also incorporate an xe2x80x9coutput endxe2x80x9d annular reflector, or an xe2x80x9cinput end annular reflector, or both.
The output end annular reflector has a wide apertured end which circumferentially surrounds the collimating reflector near its wide apertured end, and a narrow apertured end which circumferentially surrounds the input end of the light guide. The input end annular reflector has a narrow apertured end through which the light source is insertably positionable, and a wide apertured end which circumferentially surrounds the collimating reflector near its narrow apertured end. All of the reflectors are cylindrically symmetrical about a common longitudinal axis. Light rays emitted by the light source which pass to the collimating reflector are reflected by the collimating reflector and produce an output light beam having a beam width which varies as a function of distance along the aforementioned axis. The light guide""s input end is positioned at a selected distance along the axis at which the output light beam has a minimum beam width value.
Advantageously, the input end annular reflector further has a curved surface, such that, in any cross-sectional plane containing the aforementioned axis, the curved surface defines a first arc on one side of the axis, and, a second arc on an opposed side of the axis. The first arc has a first centre of curvature located on the one side of the axis, and further located within a cylinder which is symmetrical about the axis and which has a diameter not greater than the diameter of the collimating reflector""s narrow apertured end. The second arc has a second centre of curvature located on the opposed side of the axis and further located within the aforementioned cylinder.
The collimating reflector is preferably an off-axis paraboloidal cross-section, cylindrically symmetrical reflector having a focal point f. The output end annular reflector preferably forms a spherical arc section having a center of curvature near the collimating reflector""s focal point f. The light source is typically a metal halide light bulb having a light emitting arc having one end near the focal point f and an opposed end near the narrow apertured end of the collimating reflector.